The cost of photovoltaic (PV) power plants is still high while energy conversion efficiency remains low. Besides the fact that the output power that solar panels can deliver varies with solar radiation, it also depends of how power is being transferred to the load. In other words, solar panels can deliver the maximum power only at specific working points of their V-I characteristics.
Maximizing the power delivered by PV panels under changing load conditions may be desirable. During the last several decades, several methods have been proposed to track the maximum power point of a PV panel array [1], [2], [3]. Most of them are based on a “perturb and observe” (P&O) algorithm usually carried out by either a micro-controller or a DSP [4], [5], [6], [7], [8]. Moreover, only few methods are able to track the MPP using only one sensor [4], [7], [8]. P&O based MPP trackers are typically implemented by a DSP/μC in a feedback loop of a DC-DC converter, whose duty cycle (D) is controlled in such a way as to maximize the power derivable from the PV panel by an electrical load according to a basic scheme as shown in FIG. 1. In practice, controlling (varying) the duty cycle of the DC-DC converter is equivalent to perturb the voltage VPV and thus the power extracted from the PV panel.
The voltage across the PV panel terminals, VPV, and the delivered current, IPV, are periodically measured and converted into digital words. Then their multiplication (VPV*IPV) is executed to obtain the power currently extracted from the panel and the value obtained is compared with a previously calculated and stored value of output power in order to set the sign of increment of the duty cycle of the PWM drive signal of the power switch of the DC-DC converter that determined the observed variation of the output power. If that variation is greater than zero, the sign is kept unchanged, otherwise it is inverted.
The flowchart of such a typical digital MPPT control is depicted in FIG. 2. This control uses two sensors for measuring both the voltage at the panel terminals and the current needed for calculating the power currently yielded from the PV panel. And thus two analog to digital conversions may be needed too.
Furthermore, current measurement implies the connection of a resistance in series between a PV panel terminal and the DC-DC converter and its value is a trade off between conversion efficiency and measurement accuracy. Therefore, current monitoring may become difficult at low power rates, when the current drawn from PV panels is relatively small.
As interest is in assessing power variation rather than its absolute value, a stable approximation of the current power yield could be used if the DC-DC converter works in the so-called Discontinuous Conduction Mode (DCM), in view of the proportionality that exists between IPV and the duty cycle D in these conditions of operation of the voltage step-up and stabilization DC-DC converter. Therefore, the product between D and VPV can be used as representing approximately the current output power [7], using only one sensor, namely a voltage sensor, which does not use introduction of a series connected resistance that would penalize conversion efficiency.
Another consideration is that, if the PV generation plant is exploited as a battery charger or in any other equivalent application, including grid connection and alike, where the output voltage of the DC-DC converter can be considered constant in the short term, the input power delivered to the converter by the PV panel, is substantially proportional to the output power (because output/input power ratio of the DC-DC converter can be considered approximately constant) that can be expressed as the product between VOUT (which is considered constant) and the output current IOUT [8].
However, a current sensor is used, which suffers from the above noted drawback of needing a dissipative series resistance. Moreover, an analog to digital conversion (ADC) is needed as well, because of the digital processing (mainly multiplications and power data storage) contemplated by the MPPT control algorithm. Notwithstanding the relative simplicity of the digital processing that is used, a working frequency higher than the switching frequency of the DC-DC converter may be provided. Moreover, the ADC can represent a crucial part of the whole design, as its resolution strictly relates to circuit complexity and control accuracy. Some P&O based, analog MPP trackers have been disclosed in literature [9], [10]. In these articles, an analog multiplier is used to evaluate the power (VPV*IPV), eliminating the need of analog to digital conversion hardware; however two sensors may be needed.
In [10], a constant current, proportional to the power delivered by the panel(s), to the DC-DC converter, charges a capacitor during a first portion of the sampling period and discharges it during a second portion of the sampling period; then, in the third and last portion of the sampling period, the amount of charge left in the capacitor feeds a comparator whose output provides a digital signal indicating whether the power is increasing or decreasing.